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The crystal structure of methyl α-D-mannopyranosyl-(1→3)-2- O -acetyl-β-D-mannopyranoside monohydrate, C 15 H 26 O 12 ·H 2 O, ( II ), has been determined and the structural parameters for its constituent α-D-mannopyranosyl residue compared with those for methyl α-D-mannopyranoside. Mono- O -acetylation appears to promote the crystallization of ( II ), inferred from the difficulty in crystallizing methyl α-D-mannopyranosyl-(1→3)-β-D-mannopyranoside despite repeated attempts. The conformational properties of the O -acetyl side chain in ( II ) are similar to those observed in recent studies of peracetylated mannose-containing oligosaccharides, having a preferred geometry in which the C2—H2 bond eclipses the C=O bond of the acetyl group. The C2—O2 bond in ( II ) elongates by ∼0.02 Å upon O -acetylation. The phi (φ) and psi (ψ) torsion angles that dictate the conformation of the internal O -glycosidic linkage in ( II ) are similar to those determined recently in aqueous solution by NMR spectroscopy for unacetylated ( II ) using the statistical program MA′AT , with a greater disparity found for ψ (Δ = ∼16°) than for φ (Δ = ∼6°). 

The crystal structures of 2,3,4,6-tetra- O -benzoyl-β-D-galactopyranosyl-(1→4)-1,2,6-tri- O -benzoyl-β-D-glucopyranose ethyl acetate hemisolvate, C 61 H 50 O 18 ·0.5C 4 H 8 O 2 , and 1,2,4,6-tetra- O -benzoyl-β-D-glucopyranose acetone monosolvate, C 34 H 28 O 10 ·C 3 H 6 O, were determined and compared to those of methyl β-D-galactopyranosyl-(1→4)-β-D-glucopyranoside (methyl β-lactoside) and methyl β-D-glucopyranoside hemihydrate, C 7 H 14 O 6 ·0.5H 2 O, to evaluate the effects of O -benzoylation on bond lengths, bond angles and torsion angles. In general, O -benzoylation exerts little effect on exo- and endocyclic C—C and endocyclic C—O bond lengths, but exocyclic C—O bonds involved in O -benzoylation are lengthened by 0.02–0.04 Å depending on the site of substitution. The conformation of the O -benzoyl side-chains is highly conserved, with the carbonyl O atom either eclipsing the H atom attached to a 2°-alcoholic C atom or bisecting the H—C—H bond angle of an 1°-alcoholic C atom. Of the three bonds that determine the side-chain geometry, the C—O bond involving the alcoholic C atom exhibits greater rotational variability than the remaining C—O and C—C bonds involving the carbonyl C atom. These findings are in good agreement with recent solution NMR studies of the O -acetyl side-chain conformation in saccharides.